Hemophilia is a genetic disease characterized by a blood clotting deficiency. In hemophilia A (classic hemophilia, Factor VIII deficiency), an X-chromosome-linked genetic defect disrupts the gene encoding Factor VIII, a plasma glycoprotein, which is a key component in the blood clotting cascade. Human Factor VIII is synthesized as a single chain polypeptide, with a predicted molecular weight of 265 kDa. The Factor VIII gene codes for 2351 amino acids, and the protein has six domains, designated from the amino to the carboxy terminus as A1-A2-B-A3-C1-C2 (Wood et al., Nature 312:330 [1984]; Vehar et al., Nature 312:337 [1984]; and Toole et al., Nature 312:342 [1984]). Human Factor VIII is processed within the cell to yield a heterodimer primarily comprised of a heavy chain of 200 kDa containing the A1, A2, and B domains and an 80 kDa light chain containing the A3, C1, and C2 domains (Kaufinan et al., J. Biol. Chem., 263:6352-6362 [1988]). Both the single chain polypeptide and the heterodimer circulate in the plasma as inactive precursors (Ganz et al., Eur. J. Biochem., 170:521-528 [1988]). Activation of Factor VIII in plasma is initiated by thrombin cleavage between the A2 and B domains, which releases the B domain and results in a heavy chain consisting of the A1 and A2 domains. The 980 amino acid B domain is deleted in the activated procoagulant form of the protein. Additionally, in the native protein, two polypeptide chains ("a" and "b"), flanking the B domain, are bound to a divalent calcium cation. Hemophilia may result from point mutations, deletions, or mutations resulting in a stop codon (See, Antonarakis et al., Mol. Biol. Med., 4:81 [1987]).
The disease is relatively rare, afflicting approximately one in 10,000 males. Hemophilia in females is extremely rare, although it may occur in female children of an affected father and carrier mother, as well as in females with X-chromosomal abnormalities (e.g., Turner syndrome, X mosaicism, etc.). The severity of each patient's disease is broadly characterized into three groups--"mild," "moderate," and "severe," depending on the severity of the patient's symptoms and circulating Factor VIII levels. While normal levels of Factor VIII range between 50 and 200 ng/mL plasma, mildly affected patients have 6-60% of this value, and moderately affected patients have 1-5% of this value. Severely affected hemophiliacs have less than 1% of normal Factor VIII levels.
While hemophiliacs clearly require clotting factor after surgery or severe trauma, on a daily basis, spontaneous internal bleeding is a greater concern. Hemophiliacs experience spontaneous hemorrhages from early infancy, as well as frequent spontaneous hemarthroses and other hemorrhages requiring clotting factor replacement. Without effective treatment, chronic hemophilic arthropathy occurs by young adulthood. Severely affected patients are prone to serious hemorrhages that may dissect through tissue planes, ultimately resulting in death due to compromised vital organs.
Hematomas are commonly observed in moderately and severely affected hemophiliacs. In these patients, hematomas have a tendency to progressively enlarge and dissect in all directions. Some of these hematomas expand locally, resulting in local compression of adjacent organs, blood vessels, and nerves. A rare, yet often fatal, complication of abdominal hematomas is the perforation and drainage of the hematoma into the colon, resulting in infection and septicemia. Intracranial and/or extracranial hemorrhage also represent very dangerous bleeding situations. While subcutaneous hematomas may dissect into muscle, pharyngeal and retropharyngeal hematomas (e.g., complicating bacterial or viral pharyngitis) may enlarge and obstruct the airway, sometimes resulting in a life-threatening situation that requires administration of a sufficient dose of Factor VIII concentrate to normalize the Factor VIII level.
In addition to hematomas, hemarthroses are commonly observed in hemophiliacs, with bleeding into the joint accounting for approximately 75% of hemophilic bleeding. Repeated hemorrhaging into the joints eventually results in extensive destruction of articular cartilage, synovial hyperplasia, and other reactive changes in adjacent tissues and bone. A major complication of repeated hemarthroses is joint deformity, which is often accompanied by muscle atrophy and soft tissue contractures; osteoporosis and cystic areas in the subchondral bone may also develop, along with progressive loss of joint space.
Other symptoms are often observed in hemophiliacs, including hematuria and mucous membrane bleeding. Hematuria is experienced by virtually all severely affected hemophiliacs sometime during their lifetimes, and mucous membrane bleeding is common in hemophiliacs. Bone cysts (pseudotumors) are rare, but dangerous complications of hemophilic bleeding. In many of these cases, immediate treatment is necessary.
In the early 1980s, many severely affected hemophiliacs were treated with Factor VIII concentrate about three times weekly. Unfortunately, these concentrates transmitted viruses, such as hepatitis B and/or C, and human immunodeficiency virus (HIV). In the United States and Western Europe, at least 75% of Factor VIII concentrate recipients have been reported to have anti-HIV antibodies (See, Schrier and Leung, supra). Some of these patients also developed HIV-associated immune thrombocytopenia, a very serious complication in hemophiliacs. In spite of antiviral therapy (e.g., with zidovudine and pentamidine prophylaxis), which has tended to slow disease progression, full-blown AIDS (acquired immunodeficiency syndrome) occurs at an inexorable rate in hemophiliacs infected with HIV. Indeed, this has reversed the improvement in the life expectancy of hemophiliacs, which peaked at 66 years of age during the 1970s, and has dropped to 49 years (See, Schrier and Leung, supra). The development of virus-free preparations and recombinant Factor VIII has helped control infectious viral contamination.
However, for hemophiliacs, the availability of viral-free concentrates and recombinant Factor VIII, while significant, is but part of the solution. In order to prevent spontaneous internal bleeding episodes, patients suffering from hemophilia A must consistently have serum Factor VIII levels of about 1%, and preferably 5%. Currently, the cost of viral-free concentrates and recombinant Factor VIII make it prohibitively expensive to administer the clotting factor prophylactically or on a maintenance basis. Indeed, most hemophiliacs in the U.S. do not receive recombinant Factor VIII therapy on a maintenance basis, but only receive it prior to activities or events which might cause bleeding (e.g. surgery), or as a treatment for spontaneous bleeding.
Moreover, even if cost effective preparations of recombinant or virus-free Factor VIII were available, a steady state level of Factor VIII cannot be achieved by its daily administration. At best, patients receive widely varying levels of Factor VIII. Immediately following the administration, the levels are super-physiological, while prior to administration the levels are sub-physiological. Thus, there remains a need for methods and compositions that are relatively economic, yet effective in the treatment and prevention of bleeding in hemophiliacs, particularly spontaneous bleeds. Furthermore, there is a need in the art for methods and compositions for long term delivery of clotting factors (e.g., Factor VIII) which more closely mimic the steady state physiological levels observed in normal individuals.